Fine metal particles having a particle diameter of several nanometers to several tens of nanometers show various kinds of physical and chemical characteristics that are different from those of conventional metal materials. Accordingly, it is expected that such fine metal particles can be applied in a wide range of areas such as electroconductive paste, transparent electroconductive film, light blocking filter, chemical sensor, catalyst, antimicrobial material, light guiding member, gas barrier material, light scattering/reflecting member, light diffusing member, photosensitive material, photoelectric conversion element, fluorescent material and color material. The metal particles can be formed into a metal particle dispersion in the form of ink or paste by being dispersed in a solvent.
For example, a metal particle dispersion in which metal particles comprising a metal oxide such as ITO (tin-doped indium oxide) or ATO (antimony-doped tin oxide) are uniformly dispersed in a matrix material, is used for the purpose of forming a transparent electroconductive film or a heat ray shielding film. In this case, when metal particles having a particle diameter of more than several tens of nanometers are used, visible light is scattered and thus the film looks cloudy. By using metal particles having a particle diameter of less than several tens of nanometers, it is expected that a highly transparent optical material can be obtained, which is able to inhibit light scattering.
For metal particles, it is known that the melting point is dramatically decreased by reducing the size. This is because as the particle diameter decreases, the specific surface area increases and thus the surface energy increases. By using this effect, the sintering of metal particles can be promoted at lower temperature than ever before, in the method of forming a circuit pattern by directly printing a pattern on a base material, using a metal particle dispersion. Accordingly, it is expected that it would be possible to increase productivity dramatically higher than conventional methods such as a photoresist method, and also it would be possible to form circuits on resin base materials with low heat resistance, which have been difficult to use, by printing.
As just described, fine metal particles are widely used in the form of metal particle dispersions, and fine metal particle dispersions with better dispersibility and dispersion stability have been desired.
In Patent Literature 1, as a technique to obtain excellent dispersion stability of metal oxide fine particles, a coating composition for an antireflection film is disclosed, which contains a specific type of metal oxide fine particles, a non-ionic organic dispersant, a binder and an organic solvent.
In Patent Literature 2, as an ink composition that is capable of printing an image or a letter having excellent metallic luster by an inkjet printing method, the use of metal colloid particles is disclosed, which are prepared by coating metal nanoparticles with protective colloid containing an organic compound having a carboxyl group and a polymer dispersant.
However, the methods of Patent Literatures 1 and 2 need a large amount of dispersant to disperse highly concentrated metal particles, and a further increase in dispersibility has been required.
A metal microparticle dispersion is disclosed in Patent Literature 3, which contains a specific type of metal microparticles, a polymeric dispersant having a specific polyester skeleton, and a dispersion medium.
Patent Literature 3 describes that the specific polymeric dispersant have a high effect on the dispersibility of the metal microparticles and can be easily volatilized at a subsequent burning step. However, as shown in the below-mentioned Comparative Examples, by the method of Patent Literature 3, the storage stability of the metal microparticle dispersion cannot be sufficient; therefore, the metal microparticles sometimes precipitate after a certain period of storage.